The present invention relates to a printing apparatus such as a dot matrix printer, and relates particularly to a printing apparatus capable of using recording media narrower than the maximum range of print head movement, and to a control method for said printing apparatus.
Printers today are commonly connected as an essential peripheral device to data processing terminals of various types. Printers that can be used for validation printing for printing to cut-sheet so-called validation forms, which are various width of card like recording media having rather high rigidity, or slip printing for printing various types of cut-sheet forms, in addition to journal printing using roll paper, are also commonly available. Printing speeds and print quality have also improved greatly. Compact, lightweight printers offering high functionality and high quality are also available.
It has therefore become necessary to use increasingly compact carriage motors and print heads, and motor torque continues to decline as the operating duration of the dot wires in the print head continues to increase. As motor torque decreases, however, it is necessary to also reduce ribbon travel amount relative to carriage travel amount. When the dot wire operating duration increases, it is necessary to reduce the ribbon speed as a means of preventing the dot wires from catching on the ink ribbon. To prevent the carriage speed from also dropping, however, the ribbon travel to carriage travel ratio must also be reduced. As a result, when the per-line printing area is too short to achieve the required ribbon refresh distance even during logical seeking, the print head must be controlled to move beyond the printing area by an amount equal to the difference between the printing area and the ribbon refresh distance in order to properly refresh the ink ribbon.
To increase the printing speed in wire dot printers, the gap between the platen and print head, i.e., the platen gap, is reduced to shorten the dot wire stroke and thereby improve the response characteristics of the print head.
The ink ribbon is also disposed in this narrow platen gap parallel to the direction of print head travel, and to enable cut-sheet type multiple part printing forms (validation forms) to be inserted smoothly between the ink ribbon and the platen without touching the ink ribbon, a separator called a "mask plate" is disposed between the ink ribbon and the platen. This mask plate is also fastened to the carriage for carrying the print head in a reciprocating motion, and is used to press the recording media to the platen during printing. The mask plate therefore also helps prevent smudging of the recording media by preventing the interference that can easily occur between the recording media and ink ribbon when the recording media is loaded to the specified position and when recording media is advanced, and helps improve print quality by preventing the recording media from lifting off the platen during printing.
Printing noise also increases as a result of the recording media lifting off the platen when printing to cut-sheet type recording media thinner than even validation forms. The mask plate, however, also holds such cut-sheet type recording media against the platen, and is thereby able to further suppress printing noise. As a result, a high functionality, high print quality printing apparatus can be achieved in a relatively simple printing apparatus by using either a mask plate of this type or a similar mechanism that moves with the print head, as does the mask plate, to hold the recording media to the platen.
When the recording media is narrower than the range of print head movement, however, the mask plate may travel to a position outside the recording media, i.e., to an area where there is no recording media, if the range of print head movement is increased to assure a sufficient ribbon refresh distance or to prevent coil damage when the print head heats up as a result of the conventional printer control method described above. When the print head is then moved back within the boundaries of the recording media, i.e., within the printing area of the paper, the part of the mask plate pressing the recording media against the platen commonly interferes with the edge of the recording media, resulting in damage to the recording media, interference with print head movement, and/or printing errors.
When using a variety of recording media, including cut-sheet forms, the length and width of the forms will vary, and once the mask plate moves past the edges of the recording media, the recording media can easily lilt off the platen, leading quickly to the interference problems described above. As a result, when a mask plate or similar pressing mechanism is used, it is possible to achieve high quality printing to a variety of print media, but the problems described above will also occur when printing to narrow recording media.
Printers conventionally use a "logical seeking" control method whereby the print head is moved the shortest distance between the current print head position and the printing start position, and prints by moving only through the area printed to, as a means of achieving high speed printing.
With wire dot printers and other printers which print using an ink ribbon, however, an ink ribbon refreshing operation whereby the ink ribbon is advanced a predetermined amount at a regular interval is required to sustain high quality printing. While it is possible to provide a motor only for driving the ink ribbon at a regular interval, the most common method applied in compact printers is a mechanism whereby the ink ribbon is advanced using the drive force of the carriage motor in conjunction with the movement of the carriage on which the print head is mounted.
Advancing the ink ribbon with this type of mechanism, however, greatly increases the load on the carriage motor, or uses gears with a high speed reducing ratio to reduce the load on the motor and prevent the print head from jamming. As a result, the print head must travel a relatively long distance (i.e., the ribbon refresh distance ) to advance the ink ribbon far enough to refresh the ribbon.
To reduce the frequency of occurring of the case in which the printing area in one line is shorter than the required ribbon refresh distance, this conventional logical seeking control method requires the low speed reduction ratio or the high output torque of the carriage motor, thus providing a means of achieving a sufficient ribbon refresh distance with a shorter distance carriage traveling.
In a wire dot printer, a coil for driving the dot wires is housed in the print head. To prevent heat damage to this coil, or to prevent operating problems caused by wire heating, and thus maintain print quality, a control method that changes the printing frequency per trait time (the print duty) is used. This method typically uses a thermistor or other temperature detecting means to detect the internal temperature rise of the coil or print head, or a means for detecting the coil resistance instead. When the print head temperature is determined to have exceeded a predetermined threshold value, conventional logical seeking control is adjusted to reduce the print duty, and the print head is controlled to travel from one side to the other side of the printing area. To further suppress the temperature rise, "interval drive" is used. With interval drive, movement of the print head is stopped for a predetermined period at one side of the printing area.